Process of annealing malleable cast iron



C. T. HOLCRUFT.

PROCESSOF ANNEALING IVIALLEABLE CAST IRON.

APPLICATION FILED IAN. 20, 1921.

IIQQHU,

C. T. HOLCROFT.

PROCESS 0F ANNEALING MALLEABLE CAST-IRON.

APPLICATION FILED 1AN.20,1921.

l 9422;?" l Uh l Patented July M, 1922,

3 SHEETS-SHEET 2.

ammiro/L (I. TI HOLCROFT.

PROCESS OF ANNEALING MALLEABLE CAST IRON.

`APPLlcATloN FILED 1AN.20,192|.

Patented July M, 1922.,

3 SHEETS-SHEET 3- which the following is a specification, refer-A Unir rares career carrer..

' CHARLES T. HOLCROFT, OF DETROXT, llLIICHIflG-AN.

r'aocnss .or ANNEALING mamnanrn cnsr I'aoixr.`

Maarre.

To all lwhom t may concern:

Be it known that I, CHARLES T. 'HoLcRoFT, a citizen of the United States of America, residing at D'etroit, in the county of Wayne and State of Michigan, have invented certain new and useful Improvements in Processes of Annealing Malleable Cast Iron, of

ence being had therein to the accompanying drawings. This inventionrelates to a process of annealing` malleable cast iron. d

It has for its object the production of castings inwhich the graphltizing of the carbon is conducted at amuch more rapid rate than is customary in the vprior art.

. A further object of my invention is to obviate the necessity for packing the castings in pots or boxes.

By the use of a preferred form of apparatus, l accomplish the furtherV result of ,passing the castings continuously through the annealing process under predetermined conditions of temperature during each stage of the annealing operation. I accomplish they above and other objects by the use of a furnace in which the heat is applied -directly to the castings by means of an unprotected electric heating element, the furnace being so constructed that access of oxygen during the annealing operation may be prevented. The castings entering at one end of the furnace 'in the preferred form of apparatus aire made to-progress continuously past a series of heating elements, the temperature of which is so regulated that the castings will first be raised in temperature at a substantially uniform rate per hour to the predetermined annealing temperature. They will be maintained' at annealing heat for a predetermined period, and

in their further progress through the apv paratus, the heating conditions will be regulated 'so that the castings will'be lowered in temperature at a predetermined rate per hour until the critical point of temperature has been passed, after which the castings may be immediately removed from the fur-v nace-and' cooled rapidly down tothe atmospheric temperature. 1

By this method. the expense of` providing cast iron or steel annealing pots orboxes is rendered unnecessary. The time hitherto Specification of Letters Patent.

'd consumed in 'rammed Jury 1111, 11922;..

Application led January 20, 1921. Serial No. 438,608.

are used as a packing. Slag, sand, clay, and

lime, or substantially any refractory material may be used instead of the iron oxide.

The pots or containers are' then placed into a large furnace and the heat is applied directly. The boxes are arranged in such a way that the flames may play around them as completely as possible. The flames come in from the top and pass out'at the bottom along Vthe sides. The fuel may be coke, coal, oil, or' gas, gas being preferredas more readily controlled for uniformity of heating. The pots act not only as containers to hold the castings but also to prevent oxidization.

The pots become oxidized and scale o', or

crack and become unfit for use after a few operations.

.Where iron oxide is used as a packing, a portion of the carbon in the castings unites with the oxygen of the oxide and when a casting is broken there is a white skin layer. lf the composition of the metal and the conditions were correct, a black heart casting may be produced by a proper anneal but, should the temperature' of annealing be too high, ory be continued too long, the blackf heart fracture may change to a steely fracture, showing an iron in which the precipitated carbon has returned to the combined form. lf the annealed castings are cooled too fast Aafter the anneal, or if the black heart castings, after being successfully4 made are reheated and hammered, or reheated and i quenched in water,the black heart fracture will change to Steely. Graphite should normally separate from irons hi h in carbon during and shortly after solidi cation. lThe rapid cooling in castingA prevents this separation. Reheating to a point below fusion Causes a slow. readjustment of conditions by the occurrence of the reaction that was prebide.

lrates and we get the steely fracture.

viously suppressed. If however, the annealling is continued too long, the carbon pre- Vheart castings.

It takes about six days for the ordinary American annealing operation including heating up and cooling down. Sometimes this can be shortened a little by decreasing the time of the full annealing heating, or by cooling rapidly, or drawing the containers out of the oven and dumping them whilel the castings are still at a red heat. This does not glve good quality of castings. In a relatively rapid cooling there is no time for the graphitization of cementite and much less is there time for graphitization of pearlitic cementite.l A very slow""cooling affords time for t-he graphitization not only of the cementte but also of any pearlite which may be present. If the time of the full annealing temperature is reduced, the annealing temperature must 'be made higher, but the higher heat reduces the strength and ductility of the castings. In prolonged annealing the temper carbon tends to form larger flakes and the metal may become oxidized between the grains, i. e., burnt The second method of annealing is by the use of a muflle type furnace. The castings are piled inside the mufe and the heat is radiated through the mufle. The purpose of the muille is to eliminate oxidization. The

time cycle required, owing to the large mass,

is from five to ten days. The average chargev is approximately twenty tons. The method of heating both types of furnaces is either by coal hand-fired, or Stoker-fired, by coke, byl oil, or by gas, any of these requiring the attention of aman to regulate the temperature at various intervals during the process. This, ofcourse, makes a very uncertain element, as it is impossible to maintain close re lation of temperature' on large furnaces, uslng the above mentioned fuels.

In the drawings;

Figure 1 ,is a horizontal section of the preferred form of furnace employed in carrying out my invention, parts being broken away;

Fig. 2 isa central vertical longitudinal section of the furnace, correspondingparts being broken away;

Fig. 3- is a transverse vertical section on the line III- III of Fig. l;

Fig. 4 is a vertical section on the line 117.-

IV of Fig. 1;

Fig. 5 is a vertical section on the line V-V of Fig. 1;

Figs. 6, 7 and 8 are, respectively, a horizontalI section, a longitudinal vertical section and a transverse vertical section of another type of furnace which may be used in carrying out a modified Vform of my invention.

Referring to Figs. 1 to 5, I have shown an annealing chamber or leer of the continuous type consisting of three sections, A, B and C, in which an outer metallic casing 1, preferably of steel surrounds all three sections of the furnace. The sections A and B are preferably formed of an inner portion 2 of fire-brick or other suitable heat resisting material and outer portions 3 of heat insulating brick. In the section C, the outer layer be varied'to suit varying conditions. First,

the quantity of current may be made to vary leither in the entire system or in any section of the resistance elements, sincethe sections will usually be arranged for independent control. Second, the voltage may also be varied without change in quantity of current. Third, the time during which a casting is exposed to any predetermined temperature is also capable of regulation either by varying the speed of the conveyor, or by varying the length of the chamber. And fourth, the number of resistance elements in use' may be made greater or less at any portion of the period of travel of the material. Since the specic construction of the regulating and indicating mechanisms forms no part of the present invention, it has not been deemed necessary to illustrate any of the many types which may be used, but the procedure will be similar in each case. position of material will require a temperature range and a period of time during Each different comwhich each portion of the annealing operation is accomplished, which will be dependent upon the formula. To determine the time and .temperature factors a sample lot of the castings will be exposed in a furnace having a recording pyrometer system. As the temperature rises, a point will be reached at which a continuous expenditure of energy in heating will not be accompanied by a corresponding rising in temperature. This indicates that a temperature has been reached at which some chemical change is occurring. Either the voltage or the quantity of current or the number of elements in annealing will occur while the temperature falls.

. Having made the test and noting the indications of the pyrometer record, it will then be easy to construct a controlling mechanism, the nature of which will be dependent upon the type of apparatus used, which will control the quantity of current, its voltage, the number of elements in use, or the rate of speed of the conveyor for any given length of furnace, so that the conditions determined during the preliminary test may be mechanically repeated by the automatic controlling mechanism. An endless conveyor 7 supported upon wheels 8 is used to carry the articles to be annealed lengthwise of the annealing chamber. The wheels 8 roll upon rails 9 carried upon a supporting frame work 10, which also' forms the supportl for the entire annealing chamber. As shown in Figs. 3 to 5, the chamber is preferably surrounded by a pit 11 having cement walls and floor, suliicient space being provided between the pit walls and the sides of the chamber to permit workmen to have ready access to the space beneath the annealing chamber. Secured to the under portion of the supports 10 are flanged tracks 12, upon which the endless conveyor is supported on its return journey beneath the yannealing chamber. Upon the sides of the sections of the endless conveyor are-flanges 13, 4which extend into troughs 14 containing sand or other refactory material for the purpose of maintaining a substantially air tight seal which not only prevents the passage of air but also acts as an insulating member to prevent loss of heat.

The movement of the endless conveyor 7 may be continuous or intermittent. Thel h material enters at the left hand end in 'Figs 1 and 2 and the temperature is raised in section A at the rate of about 160o F. per hour until a temperature of about 1650 F. is reached, this, of course, requiring about ten hours. The material will be held at the temperature of about 16500 F. for a period of twelve hours to permit substantial completion of the chemical change of thecombined carbon in the cementite of the castings into graphite. More than ninety per cent of the entire changewill occur at this temperature.v The temperature will then be reduced at the rate of 12%o per vhour for about A twentythree hours, or to a point between, v 13.60 and 1365o F. andthen will be reduced -at'the rate of approximately 25 per hour for about fifteen hours. The castings will now beh reduced to a temperature of .about 1,000o F. and the complete cycle of malleabilization will have been completed. During this lowering of temperature, the chemical changes that were most rapid at the upper temperature continue to Athe practical limit desired. The castings may then be cooled as rapidly as possible. rllh'is makes the total time required for the process about sixty hour as compared with the present practice of about one-hundred twenty hours or more.

The above figures represent the treat- `ment given to malleable'l iron of a certain specification which represents the longest average cycle required. The complete operation has been done in forty-four hours. The time and temperature factors are entirely dependent upon the chemical analysis of the material. y y

While the castings are traveling through the sections B ofthe annealing chamber, comparatively little heat will be lost because of the prese-nce of the heat insulating brick 3, but in the section C, the rate of cooling will be more rapid because the layer of brick 3 has been omitted and there is only a layer of irebrick 2 inside the metallic casing 1. Since the cooling is divided into two p0rtions of unequal rate, the relative lengths ofv the periods being substantially the same regardless of the composition of the material,

the relative lengths of the two sections B and conducted by the metal is a function of the composition, the rate at which the initial eating may proceed will depend upon the di'erences in temperature attained vby the leastexposed or most remote portion as compared with the portion receiving the greatest heat. So long as too great dierences in temperature are avoided, the heating may be made to proceed very rapidly. 'llhus light or thin castings may be heated more rapidly than thick ones. y

By the use of an air tight metallic casing,

vno alirpcan reachthe interlor of the annealing chamber ythrough the side Walls, and by the use of electric heatin elements no gases are introduced during -t e heating operation. ny oxygen which enters as the castings are introduced-into the chamber may be caused to combine With oil, coke or Wood within the chamber so that during the annealing cycle, a neutral or non-oxidizing atmosphere Iis maintained.

In the modified forml of annealing cham- It has been found that my present process ber shown in Figs. 6, 7 and' 8, the material isloaded upon a-car 15 running on a track 16. The car is placed in a chamber 17 similar to theI section A of the continuous type of chamber illustrated in Fig. 1. This chamber has an outer casing 1, an inner layer of fire-brick and an intermediate insulating" layer in every respect identical with the correspondin annealing chamber. The air seal 13--14 is of the same type and the resistance ribbons 5'have an automatic controlling mechanism of the same neral type as is provided for the heating e ements in the first form of annealing chamber. With an annealing chamber of this `modified type, the variations in temperature will occur throu h the action of the automatic control mec anism while the castings remain stationary instead of traveling continuously or intermittently as in the previous structure. The same gradual increase of temperature, the maintenance of the annealing temperature for a predetermined period and the gradual decrease in temperature may be accomplished in this form of chamber just as in the other, the

lonly essential difference being the much smaller capacity of the modified form.

In the annealing furnaces of the type now in general use, after the annealing operation has been completed, the heavy door of firebrick which closes one end of the furnace is removed and during a period of about fortyeight hours, the temperature within the furnace 'is so great that workmen cannot enterv to remove the pots containing the castings.

Besides this loss of time and also the loss of the heat latent in the furnace walls, after anew batch of material has been set in place in the furnace, a considerable time is required, as well as the expenditure of a considerable amount of fuel, to raise the furnace to the temperature it had when the previous annealing operation was completed.

my process, I avoid the loss of heat,

bot-h at the beginning and at the end of the and a new batch of material already loaded.

upon another car may be placed immediately in the hot furnace.

A 'further saving is effected because in the lpresent process lwhere no packing is necessary all of the labor hitherto employed in loading the pots and packing them is done away with, and this in commercial practice vrepresents a very considerable part of the cost of the annealing operation. Also the cost of the pots aswell as thelabor of handling them is Saved.

structure of the continuous type structure of the material is more uniform and does not have the high carbon skin commonly found in castings annealed in mill .be contained 'in the packing materials. The

scale or the other mixtures used for packing.`

I claim:

1. The method of annealing malleable iron castings which consists in exposing the material to temperatures variable in a delinite progression, such progression being effective to raise the temperature of the material at a rate proportionate 'to the thermal conductivity to a predetermined point of reaction, to maintain reaction temperature for a predetermined time, and then to lower the temperature to a point below which rapid cooling will not injuriouslyaffect the material. l

2; The method of annealing malleable iron castings which consists in exposing the material in an atmosphere substantially devoid of oxygen to heat for a period during which the temperature of the castings rises to a degree above that at which conversion of combined carbon into graphite begins and thereafter so controlling the temperature that the desired chemical changev will occur before the material has lost suiiicient heat to render further chemical change negligible. 3. The method of annealing malleable iron castings which consists in exposing the material for a predetermined length of time in an atmosphere substantially devoid of oxygen to heat, the temperature 'lirst being raised at a rate proportionate tothe thermal conductivity of the material to a higher point than that at which the chemical changes begin, the temperature being maintained at this high point until a predetermined amount of chemical change has occurred, and the temperature then being gradually lowered at a rate suchthat thedesired amount of chemical change shall have been completed by the time the temperature has reached a point at which further chemical change is negligible.

4. The method of annealing malleable iron castings which consists in exposing the material in an atmosphere lsubstantially devoid of oxygen to heat, the temperature during the first period of exposure'being increased at a rate substantially proportionate to the thermal conductivity of the material to a point higher than that at which the desired curred, and the temperature during a third period being lowered irstslowly while further chemical change proceeds and then more rapidly as the rate of chemical-change decreases, to a point at which the desirable qualities of the material will not be injuriously affected by rapid cooling.

5. The method of operating an electrically heated furnace for annealing malleable iron electrical conditions of some of the heating elements so, that the temperature will rise at a predetermined rate to a degree above that at which conversion of combined carbon into graphite .begins and thereafter controlling the electrical conditions of some of the heating elements so that the temperature may be maintained substantially uniform for a predetermined period and then controlling the electrical conditions of some of the heating elements to cause a gradual decline in temperature for a predetermined period.

6. The method of operating an electrically heated furnace provided with means whereby an atmosphere substantially devoid of oxygen may be maintained therein for annealing malleable iron castings, which consists incontrolling the electrlcal conditions of some of the heating elements so that the temperature will rise at a predetermined rate to a point above that at which conversion of combined carbon into graphite begins and thereafter controlling the electrical conditions of some of the heating elements so that the` temperature may be maintained substantially uniform for apredetermined period and then controlling the electrical conditions of some of the heating elements. to cause a gradual decline in temperature for a predetermined Y period.

7. The method of operating an electrically heatedv furnace for annealing'malleable iron Acastings which consists in causing material to progress through the annealing chamber within a predetermined time and during its progress controlling the electrical conditions in the heating elements, so that the temperature will first be raised at a rate substantiallyproportionate to the thermal conduc- 50 tivity of the material to a point above that at which the combined carbon begins to change into graphite, and then will be maintained at a substantially uniform temperature for a ually lowered to a point at which the esirable quantities of the material will not be injuri'ously a'ected by rapid cooling.

8.l The method of operating an electrically heated furnace forannealing malleable iron castings, which consists'in moving the material past the .electrical heating elements and simultaneously controlling the movement of the material and the electrical conditions in the heating elements lso that for a predetermined length of 'timethe material will be castings, which consists in controlling the' predeterr'ninedv period and then will be (grad-e exposed to heat sucient to cause a rise in temperature at a rate substantially-proportionate to the thermal conductivity of the material until a degree of temperature has been attained above that at which conversion of combined carbon into graphite begins, and thereafter controlling the electrical conditions of the heating elements during a predetermined portion of the travel of the material, so that the temperature of the material will be maintained for a predetermined time at a degree at which a predetermined amount of chemical change occurs, and thereafter cooling the material at a predetermined rate until the temperature has fallen below a point at which the desirable qualities of the material will be injuriously affected by rapid cooling.

' 9. In the art of producing malleable castings by an annealing process wherein the material is automatically subjected to heat treatment in a predetermined and definite cycle relative to temperature and time values, and wherein the heating of the treatment chamber is responsive to the activity of a control element individual to the specific nature of the material, the method of producing the control element which consists in subjecting a sample ofthe material to tests to ascertain the heat and time factors required to-cause the desired chemical changes and then adapting the control element to cause the material to be subjected to predetermined temperature variations during predetermined periods of time.

10. The method of operating an electrically heated furnace for annealing malleable iron castings, which consists in subjecting a sample casting having the same formula as the castings to be annealed, to heat treatment in a `device having'a lrecording pyrometer indieating the heat conditions existing during the treatment, and upon the basis of data obtained from the treatment of the sample casting, preparing a controlling means HO adapted forv use in the controlling mecha'- nism for the annealing chamber whereby the electrical heating elements will be controlled in such a manner that castings ex- MDD posed to the heat of the heating elements are first raised in temperature at a predetermined speed to a predetermined point, then maintained at that point yduring a predetermined period and thereafter permitted to cool at a predeterminedrate of speed.

11. The method of annealing malleable castings which consists in ascertaining the conditions of temperature and time required duringV the annealing ofl a sample casting vand thereafter subjecting other material of 12o the same formula to similar heat'treatment with the heat automatically controlled as to degree and duration to produce a predetermined progression of temperature values during a predetermined time.

12. The method of annealing malleable chemical change specific to the castings uniron castings which consists in exposing the der treatment. 10 material to heat under conditions of control In testimony whereof I aix my signature such that during a definite time cycle a pro- -in presence of two Witnesses.

5 gression of temperatures shall occur in the CHARLES T. HOLCROFT.

materia-1 corresponding to the thermal con'- Witnesses: ductivity, temperature of conversion of com- HIRAM L. RITTS,

bined carbon into graphite, and rate of KARL H. BUTLER. 

